The technique of forming an image by the diffusion transfer principle employing a silver salt such as a silver halide is well known. The technique comprises treating an imagewise exposed photosensitive silver halide emulsion layer with an alkaline aqueous solution containing a developing agent and a silver halide solvent. The exposed silver halide grains are reduced to silver by the developing agent and the unexposed silver halide grains are converted by the silver halide solvent to a diffusible or transferable silver complex salt. This silver complex salt is diffused and transferred by inhibition to a silver precipitating agent-containing layer (image receiving layer) which is superposed with the emulsion layer to thereby cause the silver complex salt to be reduced by the developing agent in situ with the aid of the silver precipitating agent.
This technique is generally practiced using a film unit which consists of a photosensitive element comprising a support with a photosensitive silver halide emulsion layer disposed thereon, an image receiving element comprising a support with a silver precipitating agent-containing image receiving layer disposed thereon, and a processing element comprising a rupturable container containing an active alkaline aqueous solution containing a developing agent, a silver halide solvent and a rheology modifier. In practice, the silver halide emulsion layer of the photosensitive element is first imagewise exposed and, then, the photosensitive element and image receiving element are superposed in such a manner that their emulsion layer and image receiving layer will be brought together and passed over a pair of rollers in a manner that the processing element is ruptured to release the viscous aqueous solution in the nip zone of the roller pair. The film unit is then allowed to stand for a predetermined time, after which the image receiving element is separated from the photosensitive element so as to provide a print carrying the desired image in the image receiving layer. In such a diffusion transfer process using a silver salt such as silver halide, the silver image obtained is generally not black, but rather assumes a brown or other unacceptable color. It is common practice to use a toning agent so as to overcome this disadvantage of the diffusion transfer process.
An example of a well known toning agent is 1-phenyl-5-mercapto-1,2,3,4-tetrazole. S-substituted pyrimidine derivatives such as those described in U.S. Pat. No. 3,756,825 are known and have been reported to help obtain dark blue images with high optical densities.
The color of the silver image has an important bearing on the quality of the print. The colors of prints available today on the market are not necessarily satisfactory and improvement in this respect has been desired. To this day a toner has not been available that will yield an image of lacquer black shade with a minimum of metallic gloss and high maximum density.
In view of the above circumstances, intensive research has been conducted to develop a method for improving the color of the silver image with the use of a new toner and found that the use of a nitrogen-containing heterocyclic mercaptan compound having a sulfo- or carboxyl-substituted alkyl group or a sulfo- or carboxyl-substituted aryl group enables one to overcome the above-mentioned disadvantages of the prior art.